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Bonding to Existing Concrete

posted by Bob Monday, November 22, 2010 at 12:00 PM

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

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409 USER COMMENTS

I bought a house where the back patio is approx. 10ftx12ft they used expansion joints in it and didn't remove the framing boards. I now have grass growing in the expansion joints and between the patio and house. can I bond over this or should I tear it up and replace the whole patio? thanks for the help
- dawn
Thursday, October 16, 2014 at 10:44 AM

Mike S, I want to start by asking a few questions. 1- How large is the area? 2- A big concern is if the crack is a moving crack and with the sinking of the slab there may be an underlying issue that a professional may need to determine the cause and resolve before moving forward. . If the crack continues to move applying a continuous overlay that bridges the crack would not be recommended. It would again crack. An option would be to treat the crack as an expansion joint and after leveling the slab put a urethane caulk in the joint between the slab and tiles. Preparation before applying the repair material: Preparation is critical. As a first step, prep the surface by removing any foreign material such as paint, adhesives, stain, sealer, dirt, dust, loose material, and anything that will inhibit the new repair material from bonding to the old concrete. The substrate should be solid, sound and clean. Sakrete Sand Mix is recommended for areas requiring ½” up to 2”. I suggest using Sakrete Bonder and Fortifier as an admixture with the Sand Mix for a superior bond and added strength (50:50 with water). Also priming the substrate with the Sakrete Bonder Fortifier would also help with a good bond of the Sarkete Sand Mix to the old slab. Use Sakrete Top ‘N Bond to finish. Sakrete Top ‘N Bond is used in areas requiring ½” down to a feathered edge, it is polymer modified for excellent adhesion and does not require additional bonding agents. For tiling TCNA (Tile Council of North America) recommends a find broom finish to assure a good bond of the tile setting mortar to the substrate.
- Chris Technical Services
Thursday, October 16, 2014 at 8:20 AM

have a slab in utility room along center of room cement cracked and sank 1 3/4 in. Several years ago. Want to install ceramic tile . Original flooring was stick down tiles and left residue like tar but I'd not tacky. I need a product that can be graded 2 in to 0. I need advice project has been at stand still for some time!
- problem floor mike s
Tuesday, October 7, 2014 at 11:16 PM

RMHoffmann, thank you for using our products. Sakrete Sand Mix may be the best product to use to level out the areas in question. Sand Mix is used for areas requiring ½” up to 2”. We also recommend using Sakrete Bonder and Fortifier as an admixture to provide a strong bond to the existing concrete. Make sure the area is clean and free of any dirt, loose material, paint, stain, etc. anything that will inhibit the bond with the existing concrete.
- Chris Technical Services
Monday, October 6, 2014 at 10:39 AM

Cynthia, you will need to wait the 28 days for the concrete to fully cure first. Rebar will need to be used to join the addition to the existing.
- Chris Technical Services
Monday, October 6, 2014 at 10:04 AM

We just poured a mow strip for a fence so it is 3 1/2 thick and about 8 inches wide however there is about 20 ft that we poured to low in the ground so now we have to pour another 2 to 2 1/2 inches to raise up the mow strip to grass level, it would be best to feather it but I am thinking of cutting the existing concrete down an inch and a half to get a better looking start, It will have to gradually get up to the 2 1/2 inches. We just poured this concrete about a month ago if that long ago. What is your recommendation as how to accomplish this task, by the way we did use your product, 120 bags of it. Thanks
- RMHoffmann
Wednesday, October 1, 2014 at 6:04 PM

We just poured a mow strip for a fence so it is 3 1/2 thick and about 8 inches wide however there is about 20 ft that we poured to low in the ground so now we have to pour another 2 to 2 1/2 inches to raise up the mow strip to grass level, it would be best to feather it but I am thinking of cutting the existing concrete down an inch and a half to get a better looking start, It will have to gradually get up to the 2 1/2 inches. We just poured this concrete about a month ago if that long ago. What is your recommendation as how to accomplish this task, by the way we did use your product, 120 bags of it. Thanks
- RMHoffmann
Wednesday, October 1, 2014 at 5:57 AM

Our contractor poured our front porch steps a few days ago. Unfortunately, he made them 6 inches too short in width. Is there a way to extend the concrete steps the 6 inches that we need?
- Cynthia
Monday, September 29, 2014 at 2:24 PM

Elaine, yes bonding to existing cured concrete (at least 28 days) is no problem. First prep the area by removing any dirt, debris, paint, stain, sealer, loose material, or anything that would inhibit performance of the new repair material. To determine what material would be best for this repair I need to know the maximum depth that will be applied. Sakrete Top'n Bond is used for repairs requiring up to ½” down to a feathered edge. Sakrete Sand Mix can be used for repairs from ½” up to 2” in a single lift.
- Chris Technical Services
Tuesday, September 23, 2014 at 3:28 PM

I have just had new handicap parking with flare-sided curb ramps cut into the existing sidewalk at my shopping center. The running slope of the curb ramp is not correct and will need to be built up to achieve the correct curb ramp surface level to be complaint with ADA laws. Do you have a product that will bond with the new concrete that will allow me to build up the curb ramp? Is there a chance that the product will crack?
- Elaine
Monday, September 22, 2014 at 4:27 PM

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